feat(metering): add DFS stats collector for write requests and bytes

[JmPotato]

What problem does this PR solve?

Issue Number: ref #9707.

What is changed and how does it work?

Add `dfs-stats` collector to track DFS write requests and written bytes per keyspace/component.

Check List

Tests

• Unit test
• Integration test

Release note

None.

[codecov]

Codecov Report

❌ Patch coverage is 80.83333% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.79%. Comparing base (9b18ac3) to head (f14d764).
⚠️ Report is 718 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9783      +/-   ##
==========================================
+ Coverage   73.71%   76.79%   +3.07%     
==========================================
  Files         440      490      +50     
  Lines       48866    78166   +29300     
==========================================
+ Hits        36024    60030   +24006     
- Misses       9769    14473    +4704     
- Partials     3073     3663     +590     

Flag	Coverage Δ
unittests	76.79% <80.83%> (+3.07%)	⬆️

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: okJiang, rleungx

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-09-28 02:19:09.832283782 +0000 UTC m=+755159.902777462: ☑️ agreed by rleungx.
• 2025-09-28 03:50:44.93680142 +0000 UTC m=+760655.007295102: ☑️ agreed by okJiang.

[ti-chi-bot]

@JmPotato: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
non-block/pull-unit-test-next-gen	f14d764	link	false	/test pull-unit-test-next-gen

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